The present invention relates to a semiconductor laser module and in particular to a semiconductor laser module which is preferable for use for an semiconductor laser module with an external resonator.
The structure of a prior art semiconductor laser module with an external resonator will be described with reference to FIGS. 3 and 4. FIGS. 3 and 4 are schematic views explaining prior art semiconductor laser modules with an external resonator.
Prior art technique to oscillate a semiconductor laser element at a wave length in a narrow band range comprises using an external resonator which is formed between a reverse side (HR side) of the semiconductor laser element 1 and a fiber grating 7 by providing an optical fiber 5 with the fiber grating 7 having a diffractive grating in a semiconductor laser element 1 oscillatable wave length range as shown in FIG. 3 or using an external resonator which is formed by feedbacking the light which has transmitted through a band-pass filter 4 which transmits light having a predetermined wave length in a semi conductor laser element 1 oscillating wave length range to the semiconductor laser element 1 by means of a reflecting point 8 which is deposited on the tip end of a pig-tail fiber by an HR coat as shown in FIG. 4.
However, the above-mentioned prior art has the following problems. The approach using the fiber grating 7 which is shown in FIG. 3 has a problem in that the central wave length of the module is shifted due to changes in the environmental temperatures. The reason resides in that the central wave length at 50xc2x0 C. is shifted forward the longer wave length side by about 0.75 nm in comparison with that at 0xc2x0 C. in case where the fiber grating 7 is used under an environmental condition of 0-50xc2x0 C., since the central wave length of the fiber grating 7 usually has a temperature dependency of about 0.015 nm/xc2x0 C.
The approach for reflecting the transmitted light on a reflecting point 8 which is provided at the tip end of the pig tail fiber as shown in FIG. 4 requires optical components including collimator lens 9, 10 to take the transmitted light from the reflecting point 8 to an optical fiber 5 which is provided at the tip end thereof. Even if these optical components are adjusted at a high precision, an inevitable loss of about 0.5 dB occurs. As a result, it is difficult to achieve a high output of the module and a high reliability of the element.
The present invention has been achieved in view of the above-mentioned problems. It is a primary object of the present invention to provide a semiconductor laser module which is capable of achieving the stabilization of the central wave length and a low coupling loss.
In order to achieve the above-mentioned object, the present invention provides a novel semiconductor laser module. The semiconductor laser module comprises: a semiconductor laser element emitting laser light, an optical fiber guiding the laser light, an optical system condensing the laser light onto the optical fiber, and a band-pass filter which is held by a holding device disposed between the semiconductor laser and the optical fiber transmitting therethrough only the light having a predetermined wave length. The laser module comprises a reflecting element disposed within a wave guide of the optical fiber for reflecting part of incident laser light so that a resonator is formed between the reflecting element and a laser light emitting end face of the semiconductor laser element.
In the present invention, it is preferable that the holding device for holding the band-pass filter includes a mechanism for tilting the band-pass filter at a desired angle relative to the direction of travelling of the laser light emitted from the semiconductor laser element, and in that the reflecting element which is formed in the waveguide of the optical fiber is formed of either a structure of cores having different refractive indices which are connected to each other or a structure in which part of the core is doped with a material modifying light refractive index (e.g., a material of high refractive index).
In the present invention, it is preferable that the reflecting element is separated from the laser light emitting end face of the semiconductor laser element at a distance which is 50 cm or more.